When measuring optical radiation it is important to know the basic characteristics of the source including spectral intensity distribution, spatial distribution, absolute intensity levels and temporal behavior. Source spectral intensity distribution can be monochromatic, near monochromatic, continuous spectra, band or line spectra.

Typical monochromatic sources are lasers and output signals from narrow bandwidth monochromators. Radiation from this type of source can be measured with unfiltered semiconductor detectors provided the spectral sensitivity at the wavelength of the monochromatic radiation is known through calibration. Low pressure Hg sources are also considered monochromatic since more than 95% of the output is emitted at the 253.7nm wavelength.

Examples of near monochromatic sources include light emitting diodes (LEDs) or optically filtered sources. If the radiation is within a relatively narrow band and has an intensity distribution approximating Gaussian, spectrally calibrated semiconductor detectors without correction filters may be used. If the bandwidth is broad or if the intensity distribution is asymmetrical, a 'flat' radiometric detector response is required. The detector response should be flat over the entire wavelength range of interest with steep slopes at or near the cut-on and cut-off wavelengths.

Sources covering a relatively large range of wavelengths without gaps are known to have a continuous spectrum. Typical sources of continuous radiation spectra include halogen lamps. Band spectrum sources display gaps separating individual regions of radiation. If a source spectrum emits a number monochromatic signals at various wavelengths, it is called a line spectrum. Filter-corrected semiconductor detectors are quite suitable for the measurement of both continuous spectra and band spectra, but spectral mismatch errors can occur which must be considered and corrected for. Spectral errors will greatly affect measurement accuracy when sources with lamp spectra different than the source used in the calibration are measured. In spite of the large and linear measurement range of semiconductor detectors, it is still necessary to pay particular attention to the intensity level of the source. The range of photocurrents exhibited by the best semiconductor detectors span from femtoamps to milliamps. Linear resolution is limited by noise at the lowest currents and saturation at higher currents. With a typical spectral sensitivity of 0.1 to 0.6 A/W, these detectors are better suited to the measurement of low intensity radiation. When measuring high intensities, appropriate attenuation elements should be added to the detector. The temporal behavior of optical radiation can be classified as: CW radiation (continuous wave), modulated radiation and pulsed radiation.

 Please Contact us to discuss your requirements together.